Influence of Thermal Distortion on Spur Gear Tooth Contact

摘要

The increase of efficiency in automotive gear transmissions has been a key subject of research in the past decade (Ref. 1); to this aim, dip-lubricated gearboxes have been designed with minimized oil levels in search of reduced churning power losses (Ref. 2) and maximum power-to-weight ratios. However, when such gears' running conditions reach the highest pitch line velocities and torques, bulk temperatures increase rapidly, resulting in reduced oil film thickness, higher surface shear stresses and an increased number of asperity contacts—all of which may lead to a premature failure of the gear pair. Moreover, in such cases, not only high temperatures are reached but also significant temperature gradients are found between pinion and gear. Figure 1 shows a sample of the aforementioned operating case. As can be seen, the optimal conditions lie between a relative immersion depth of H/D=0.1 to 0.25, where power losses are minimal. However, with these oil levels the temperature difference between pinion and gear reach 20°C to 30°C because the small size of the pinion results in a lower convection heat transfer due to the reduced oil-lubricated area and its increased rotational speed (with respect to that of the gear) increases the amount of heat flowing inwards, resulting in a higher bulk temperature of the pinion. This phenomenon leads to relative pitch deviations and thermally induced profile distortions (Ref. 3).